This invention relates generally to filament wound vessels and particularly to an improved polar fitting or boss about which a plurality of layers of filaments are wound.
Filament wound vessels often are constructed in a spherical shape or a cylindrical shape with generally spherical end portions for use in high pressure container applications. In many circumstances, the qualities of lightweight construction and high resistance to fragmentation and corrosion damage are highly desirable with such pressure vessels. Therefore, for some years, these design criteria have been satisfactorily met by the development of high pressure containers fabricated of laminated layers of wound fiberglass filaments or various types of synthetic filaments which are bonded together by a thermal-setting epoxy resin.
Such vessels have other important advantages, including filament winding patterns which produce a quasi-isotropic composite. Such composite pressure vessels currently are used in the aerospace industry, for instance. In some applications, the vessels are required to contain ultra-high pressures, operating at 25,000 p.s.i. with design burst values in the 50,000 p.s.i. range. Composite pressure vessels are especially appropriate for such high pressure applications, since the manufacture of a thicker vessel wall structure essentially requires winding more layers of filaments. However, pressure vessels with thick walls are characterized by steep strain gradients through the wall, the inner strains being quite high when compared with those at the outer surface. For instance, an important parameter in controlling the strain gradient is the transverse or radial stiffness of the composite. Transverse stiffness is influenced by the wind angle of the vessel, as well as any delaminations or other defects induced during fabrication. Of major consideration, and thus a major problem, is possible composite buildup which results in severe bridging of the fibers.
The problem of composite buildup and fiber bridging is particularly prevalent in polar wound vessels, such as spherical vessels or other vessels having a spherically shaped end. Most often, a polar opening is provided with a fitting or boss disposed in the opening and protruding outwardly therefrom. The polar boss may comprise an integral part of a thin-walled metal liner of the vessel. The vessel is wound by a known method in a numerically controlled winding apparatus using a sequence of polar winding patterns wound with incrementally increasing wind angles away from the polar opening or fitting. The required thickness is obtained by repeating this sequence of patterns an integral number of times corresponding to the desired number of layers.
The fabrication of a thick cylindrical vessel requires special attention at the polar opening or fitting which normally is surrounded by a generally spherical portion of the vessel. The composite buildup adjacent to a conventional polar boss becomes excessive causing the curvature of the fiber path to become negative in certain regions of the spherical or dome portion of the vessel. This can result in the filaments losing contact with the surface being wound, such as a previously wound layer of filaments, and is referred to as "bridging." Bridging can cause possible premature dome failure. In addition, bridging can severely limit the vessel performance of an ultra-high pressure vessel due to the high composite porosity and resulting low transverse (through the thickness) compressive stiffness and strength. Also the composite shear strength is reduced which may be detrimental even in thin-walled vessels.
Another problem which arises due to the thickness of the spherical wall portion is that due to the low transverse modulus of elasticity of the composites, the radial strain is large in comparison with the strain in the planes of the filaments causing lower strain in the outer helical fibers relative to the inner helical fibers. As the wall thickness of the vessel increases, the share of the load carried by the outer fibers diminishes so that the burst pressure of a particular vessel has an upper limit.
The present invention is directed to solving these problems of the prior art by providing a filament wound pressure vessel with a new and improved polar fitting or boss.